The life-time of metal tools being worked under severe wear- and corrosion conditions can be prolonged by coating the tool with a wear- and corrosion resistant surface. Such metal tools are found in various industries and may be made for example of low alloyed steels, bronze, stainless steel, or cast iron.
Many industries have cast iron parts which are subjected to wear, corrosion and thermal cycling. These include hollow glass manufacturing, automotive, marine, ground engaging tools etc. An example of this is tools used for the production of glass bottles and jars. To improve the lifetime of the equipment, critical parts (e.g. mould and guide ring edges, plungers and neck rings surfaces) are coated with nickel based self-fluxing powders. Different hardfacing methods are used: for example plungers are coated by flame spray followed by fusing or HVOF followed by fusing, while mould edges, neck rings, guide rings are coated using either powder welding or plasma transfer arc (PTA)-welding.
Glass forming takes place in two steps: a pre-moulding, carried out in a so called blank mould, and final moulding, carried out in a so called finish mould. The temperature of the glass in contact with the mould is roughly 1100° C. in the blank mould and 700-800° C. in the finished mould. The temperature of the mould can fluctuate up to 550-600° C. but the surface temperature can be even higher during short time. Moreover, the surface of the mould is in contact with chemically active glass. Important requirements for a mould are good surface finish, good thermal shock resistance, resistance to dimensional change, resistance to oxidation and growth. This leads to three critical problems: sticking/adhesion of glass to the die surface and formation of irregularities (surface roughening); oxidation of the die, and; wear/pitting of the die.
These problems result in imperfections in the glass products and limit the service life of the die.
In general, self-fluxing alloys are defined as iron-base, nickel-base, cobalt-base or copper-base alloy containing 0.5-5% boron, 0.1-6% silicon and up to 3% carbon in combination with strong carbide and boride formers as W, and/or Mo and/or Cr. The most commercially used self-fluxing alloys are Ni based alloys containing boron and silicon either singly or in combination, in excess of 1.5%. Boron and silicon decrease the melting point by several hundred degrees; they promote wetting by reducing oxide of Ni, Co, Cr and Fe; they form a light low melting point boron silicate glass which flows to the surface of the deposit and protect it from oxidation. Boron is also a potent hardener forming with nickel, chromium, molybdenum, tungsten and iron hard borides. In the case of Ni-based self-fluxing alloys boron content ranges from 1-3.5%, depending on chromium content, which can be as high as 16%.
The substrate materials used in various industries, may be low alloyed steels, nodular or lamellar cast iron, bronze, or stainless steel. For some applications, lamellar cast iron is the preferred choice due to good thermal conductivity which allows for higher production rate and lower cost. Hardfacing by overlay welding of lamellar cast iron is difficult as cast iron is sensitive to cracking due to the low strength of the graphite lamellas, low ductility and thermal expansion coefficient close to zero. Crack formation may also occur in hardfacing of other types of substrates, such as bronze or stainless steel. Presently, crack free coating can be obtained only if overlay welding of cast iron parts is carried out with nickel based self-fluxing grades.
Due to cost and environmental and health concerns, it has long been a wish to reduce the alloying cost by introducing more Fe into the powders, but attempts to do so have not been successful, resulting in increased crack formation in the lamellar cast iron heat affected zone (HAZ).